1. Field of the Invention
The present invention relates to an image recording material which can be used as a master in offset printing, and particularly to a positive-type photosensitive composition for an infrared laser for what is known as direct plate making in which a planographic printing plate is prepared directly based on digital signals from a computer or the like.
2. Description of the Related Art
Systems hitherto known as direct plate making systems whereby planographic printing plates are prepared directly according to digital data from a computer include (1) a system based on electrophotography, (2) a system based on photopolymerization utilizing a combination of exposure of a photosensitive material to an Ar laser and post-heating, (3) a system using a photosensitive material prepared by laminating a photosensitive material comprising a silver salt onto a photosensitive resin, (4) a system based on a silver master, and (5) a system based on the break-down of a silicone rubber layer by means of an electric discharge or laser light.
However, these systems are associated with drawbacks. That is, system (1) based on electrophotography involves complicated processes such as electrostatic charging, exposure and development together with complicated and large-scale equipment; system (2) requires a post-heating process and a plate making material which has a high sensitivity and therefore cannot be easily handled in a well-lighted room; systems (3) and (4) involve a complicated process and lead to high costs because of the use of silver salt, and system (5) has not yet been freed from the problem of residual silicone on the surface of the printing plate, although this system has reached a relatively high level in terms of degree of perfection.
In recent years, owing to the remarkable progress in laser-related technologies, solid-state lasers and semiconductor lasers, which emit rays in regions ranging from the near infrared region to the infrared region and which have a high power output power yet are nevertheless small-sized, are easily available. These lasers are very useful as a light source of exposure when a printing plate is prepared directly from digital data of a computer or the like.
A resin such as a novolac resin, which has phenolic hydroxyl groups and is soluble in an aqueous alkaline solution, is used as a polymeric compound soluble in an aqueous alkaline solution in a conventional positive-type planographic printing plate material for use with an infrared laser in a direct plate making process. For example, Japanese Patent Application Laid-Open (JP-A) No. 7-285,275 discloses an image recording material containing additives such as a resin such as a novolac resin, which has phenolic hydroxyl groups and is soluble in an aqueous alkaline solution, and a substance which generates heat upon absorbing light together with other substances such as onium salts or quinone diazide compounds. The image forming mechanism of this image recording material consists in that the onium salts or the quinone diazide compounds act as a solubility inhibitor so that the solubility of the alkali -soluble resin is substantially decreased in image portions whereas the onium salts and the quinone diazide compounds are thermally degraded and therefore do not act as a solubility inhibitor in non-image areas.
One problem of this image recording material is that it must be handled under a yellow-light lamp because the onium salts and the quinone diazide compounds absorb light of 350 to 500 nm which is within the region of visible light. Another problem arises when a printing plate prepared from this image recording material is cleaned with a solvent such as a cleaner solution so that the printing plate may be reused, because the novolac resin has a low resistance to solvent, the durability of the printing plate deteriorates if cleaner solution is used during printing.
In order to increase the durability of the printing plate, JP-A No. 7-285,275 describes an acrylic resin or a urethane resin used in an image recording material already containing onium salts or quinone diazide compounds. However, the acrylic resin or the urethane resin is not employed to function as a solubility inhibitor which decreases the solubility of the resin which has phenolic hydroxyl groups and is soluble in an aqueous alkaline solution.